(1) Field of the Invention
The present invention generally relates to multiple channel instrumentation recorders. More particularly the system incorporates a multiplexing and demultiplexing technique for use on input limited recording devices wherein the number of channels to be recorded exceeds the number of input channels.
(2) Description of the Prior Art
The invention described herein is applicable in situations where it is necessary to simultaneously record and recover multiple channels of certain types of electrical information on a tape recorder. Such information is characterized by continuous signals whose frequency bandwidths are low compared to the available bandwidths in the tape recorder channels recording them. In addition such information may include direct current (DC) within its bandwidth.
Conventional multiple channel instrumentation recorders are normally used to record such information. These recorders normally have two modes of recording, direct record and frequency modulation (FM) record. If the information bandwidth contains DC then the FM record mode is used. FIG. IA illustrates such a configuration. Six signals are simultaneously recorded on six channels of a multichannel instrumentation recorder configured in the FM record mode. These signals are recovered by playing back the tape.
In certain applications, the number of channels of information to be recorded simultaneously exceeds the number of channels available on the tape recorder and it is impractical to use more than one instrumentation recorder to achieve the number of channels required. Multiplexing an demultiplexing techniques are then used to record more than one channel of information on a single tape recorder channel and recover it. This is illustrated in FIG. 1B. When more than one information channel is multiplexed the excess bandwidth in the tape recorder is used to accommodate the additional channel(s) of information. The demultiplexing process reverses the multiplexing process to recover the information channel(s) from the single tape recorder channel.
Frequency division multiplexing and demultiplexing is commonly used in applications where multiple information channels are recorded and recovered from a single tape recorder channel. The process is very similar to the process of recording and reproducing using a single FM tape recording channel. Each information channel modulates an FM carrier which is recorded on a direct record tape recorder channel. The modulated carrier's frequency band is limited in such a way that it does not interfere with the other modulated carriers. This limitation causes each information channel bandwidth to be limited in a proportional manner. Recovery of the information channels is accomplished by using individual FM discriminators. Each discriminator is tuned in such a way as to allow the demodulation of a single modulated carrier in a manner that is quite similar to that used by FM broadcast radio receivers.
Frequency division multiplexing and demultiplexing is an efficient way to utilize excess tape recorder channel bandwidth to record and recover additional information channels. The greatest single objection to this method is likely to be the amount of precision analog hardware required to support it. Each multiplexer channel requires precise band limiting filters and a carefully tuned FM modulator. The demultiplexer requires precise band limiting filters and a carefully tuned FM demodulator. There are many instances where it would be appropriate to trade off efficient use of tape recorder bandwidth to achieve substantial hardware simplification.